Binocular indirect ophthalmoscopes are used by an ophthalmologist to look into a patient's eye. A unit is strapped onto the ophthalmologist's head, and he holds an ophthalmoscopic lens at arm's length close to the patient's eye, as is discussed in Hovland, K. R. and Schepens, C. L., "Clinical Evaluation of the Small-Pupil Binocular Indirect Ophthalmoscope", Arch Ophthal, Vol. 82, October 1969. The unit on the ophthalmologist's head has movable mirrors in order to place images of the observer's two pupils and a light source in the patient's pupil. The head unit accommodates observers having different distances (PD) between their pupils and also has adjustment of the reducing pupil distance (RPD) mechanism in order to adjust the distance between the images of the viewer's two pupils within the patient's pupil. There also typically is movement of a mirror directing light from the illumination source to adjust the distance of the illumination source image from the viewing pupils images within the patient's pupil.
FIGS. 1-3 describe a prior art binocular ophthalmoscope system. Ophthalmologist head unit 10 includes viewing mirror assembly 12 having two reflective surfaces at approximately 45.degree. to the viewing direction along axis 14. These surfaces provide two optical paths 16, 18 perpendicular to the viewing direction. Mirrors 20, 22 are spaced from each other by the distance PD and redirect the optical paths 90.degree.. The redirected optical paths extend through ocular lenses 24, 26 to the observer's pupils 28. Ophthalmoscopic lens 30 is held close to the patient's eye 32 and provides an image at image plane 44. The distance from the viewing pupils 28 to lens 30 is known as the working distance (WD). Viewing mirror assembly 12 is moved forward or backward along the viewing direction of axis 14 (as indicated by the RPD arrows in FIG. 1) in order to change the distance between the images of the observer's pupils 34, shown with tho image of tho illumination source 35, in the patient's pupil 37 in FIG. 2. When assembly 12 is moved forward, as indicated by dashed lines in FIG. 1, the images of the viewer's pupils 34 will be further apart in the patient's pupil 37. Owing to the diverging nature of the light rays from image plane 44, in some head units 10, e.g., the S-P Ophthalmoscope sold by MIRA, Inc., Waltham, Mass., the reflective surfaces of mirror assembly 12 are adjusted angularly with forward and backward movements of assembly 12. In some other head units, e.g., the one shown U.S. Pat. No. 4,684,227, there is no such angular adjustment, with a possible resulting effect on the quality of the stereo image.
Referring to FIG. 3, showing the illumination system of a prior art binocular ophthalmoscope, mirror 38, which is separate from mirror assembly 12, is used to direct light from light source 40 and lens system 42 to the patient's eye 32. Ophthalmoscopic lens 30 provides an image of light source 40 to the patient's pupil 37. In some prior art binocular ophthalmoscopes, the illumination mirror is moved independently of the viewing mirror assembly, which typically is located underneath it. In some other prior art binocular ophthalmoscopes, the illumination mirror is fixedly mounted with respect to the viewing mirror assembly and thus moves with it.
Hubertus U.S. Pat. No. 4,838,678 describes a binocular ophthalmoscope employing plano-convex lens 19 as part of the objective lens system with lenses 28. The ophthalmoscope also includes eye-piece 34 (including an ocular lens) and Schmidt or Pechan prism 42 to fold the rays of light within it and re-invert the image.